After turning 15, Bäckström began his junior career in the 2002–03 season, playing for Brynäs IF of the J20 SuperElit league. [1] After playing parts of three seasons there, in the final of which he had 34 points in 29 games (17 goals, 17 assists), he was called up to play for the club's team in the Elitserien, Sweden's top professional hockey league. In part due to the increased competition brought about by the influx of NHL players into the league (as a result of the 2004–05 NHL lockout), Bäckström was unable to record a point.

In his second season (2005–06) his numbers were greatly improved, recording 26 points (10 goals, 16 assists) in 46 games, including a goal in four playoff games. Quickly becoming a first-line center and top player with his team, he was named both the Elitserien Rookie of the Year as well as the Swedish Junior Hockey Player of the Year for the first of two times (other notable players to win the latter include Peter Forsberg, Henrik Lundqvist, Daniel Sedin, Henrik Sedin and Niklas Kronwall).[2]

Following his successful campaign, he was drafted fourth overall by the Washington Capitals in 2006, but announced on July 10, 2006, he would wait a year before leaving Sweden to play in the NHL. This made him the fourth-highest drafted Swede, after Mats Sundin (first), Daniel Sedin (second) and Henrik Sedin (third). (Although Victor Hedman was later picked second in 2009, and Gabriel Landeskog and Adam Larsson were the second and fourth picks, respectively, in 2011.)

In his third and last season with Brynäs IF, Bäckström’s development had continued, and he saw improvements in points (12 goals, 28 assists) despite playing in one less game than the previous season. He also improved in post-season play, and was able to record six points (3 goals, 3 assists) in seven games.

On May 21, 2007, Bäckström signed a three-year, entry-level contract with the Capitals to begin playing during the 2007–08 season. His development and impressive play led some, such as HockeysFuture.com, which ranks NHL prospects, to consider him the most talented Swedish prospect in recent history. Drawing comparisons to Peter Forsberg, he was noted primarily for his vision, passing and ability to maintain possession of the puck in traffic.[3] For these reasons, he was expected to be a catalyst on the power play, and his defensive prowess and puck movement would make him an equally valuable asset when short-handed.

While Bäckström was decent early in his first season, still adjusting to the smaller ice rinks of North America, his season rapidly improved once an injury to Nylander promoted him to the first line. Playing alongside Alexander Ovechkin, Bäckström set NHL and team records while helping Ovechkin win the Art Ross Trophy and the Maurice "Rocket" Richard Trophy. These two would soon establish themselves among the premier linemates and duos in the league, with the formidability of a top flight scorer and playmaker proving to be hard to defend against.

With better talent surrounding him, Bäckström would go on to finish his rookie campaign with 69 points (14 goals, 55 assists), with three goals and 22 assists on the power play. He was named the runner-up to the Chicago Blackhawks' Patrick Kane for the 2007–08 Calder Memorial Trophy for Rookie of they Year (Kane had 1,087 votes to Bäckström's 872). The Blackhawks' other superstar rookie, Jonathan Toews, placed third in votes with 647. However, Bäckström was named to the All-Rookie Team along with Kane and Toews.[4]

In his second NHL season, Bäckström would go on to lead both the Capitals and Swedish NHL players with 66 assists, and added 22 goals for 88 points, placing him within the top ten NHL scorers in the 2008–09 regular season. Bäckström would again prove to be a force in the playoffs, tallying 15 points in 14 games.

Following the 2008–09 NHL playoffs, Bäckström was awarded the Viking Award for being the best Swedish-born player in the 2008–09 NHL season; he is only the second Capital to win this award, following behind Calle Johansson.

Bäckström played his first game with Sweden on April 6, 2006, in a game against Norway. He won the World Championship when he represented Sweden in the 2006 World Championship. As of that tournament, he is the youngest Swedish player ever in an Ice Hockey World Championship tournament. Bäckström played the last four games in the World Championship (roster spot held open for Daniel Alfredsson) and was directly appointed to the first line together with idols Henrik Zetterberg and Johan Franzén.

In a drug test at the 2014 Winter Olympics, Bäckström's A-sample indicated doping due to an allergy medication containing pseudoephedrine.[9] He was prevented from playing in the final as a result. The B-sample, analyzed after the final, also showed values above the allowed limit.[10] The International Ice Hockey Federation (IIHF) opposed the decision to stop Bäckström from playing in the final.[11] His silver medal was withheld until the International Olympic Committee (IOC) determined whether or not to award it to him.[12] On March 14, 2014, the IOC Disciplinary Commission decided that Bäckström would be awarded the silver medal. The commission confirmed the provisional suspension that rendered Bäckström ineligible for the final.[13] However, in November 2014, the World Anti-Doping Agency (WADA) appealed the IOC decision to award Bäckström his silver medal. [14] This appeal was resolved in January 2015 when Bäckström, the IOC, WADA and the IIHF agreed to a settlement in which he accepted a reprimand but was cleared of attempting to enhance his performance. [15]

Bäckström's father Anders is a Swedish hockey player who played his career for Brynäs IF, totaling ten seasons for the club. Nicklas' mother Catrin Bäckström (born in Finland) played handball in the Swedish and Finnish Elite League in the late 1970s and early 1980s. Nicklas' older brother Kristoffer is a retired professional hockey player, who played the second tier of the Swedish league system with Hammarby IF, and in a German league. On October 8, 2013, Bäckström and his girlfriend, Liza Berg, welcomed their first child (a daughter, Haley) into the world.